2016 ASHRAE Annual Conference

A panel has been developed that when exposed to the sky, will cool itself below the ambient air temperature by a mechanism known as radiative sky cooling. In this mechanism, heat is emitted from the panel’s surface to the atmosphere as long-wave infrared radiation. Since the atmosphere is transparent to long-wave infrared radiation, the panel surface is able to do heat exchange with the upper atmosphere, which is typically much colder than the ambient air temperature. Remarkably, this is an entirely passive and renewable mechanism that can be used to reject heat to the environment, even at temperatures below the ambient air temperature. Historically, this mechanism has only been accessed at night. However, we recently demonstrated that a properly designed surface can achieve the same effect during the day, making radiative sky cooling possible even under direct sunlight.

In this paper, a panel with a surface designed for radiative sky cooling is used to demonstrate the passive cooling of water below the dry-bulb temperature with no evaporative water losses, where the only energy input is to pump water. For a surface area of 0.74 m² (8 ft²), we demonstrate water cooling of 3°C (5.4°F) below the dry-bulb temperature at a water flow-rate between 6-9 L/hr (1.6-2.4 gal/hr). This corresponds to an effective heat rejection rate between 40 and 100 W/m² (13 and 32 Btu/hr-ft²).

One possible application of these panels is to serve as a modular cooling tower, replacing a traditional cooling tower in a water chiller system. This might be desired under conditions when water resources are constrained, and high efficiency cooling is required. To demonstrate the benefit of the cooling panels on a water chiller system, a thermodynamic analysis using the TMY3 dataset (typical meteorological data) from Las Vegas, NV is presented and the benefit on a typical office building’s cooling system is assessed.

Radiant cooling system has proven to be a low energy consumption system for building cooling. This study describes the use of cooling tower in radiant cooling system to improve the system efficiency. A comprehensive simulation feasibility study of the application of cooling tower in radiant cooling system was performed for the fifteen cities in different climatic zones of India. It was found that in summer, the wet bulb temperature (WBT) of the different climatic zones except warm-humid is suitable for the integration of cooling tower with radiant cooling system.  In these climates as an average 24°C to 27°C temperature of chilled water can be achieved by using cooling tower. In order to achieve the energy saving potential the three different configurations of radiant cooling system have been compared in terms of energy consumption. The different configurations are: the radiant cooling system integrated with cooling tower to provide chilled water to the floor, wall and ceiling mounted tubular installation. A variable air volume system is also coupled for the dehumidification, ventilation and additional cooling. The radiant cooling system integrated with cooling tower to provide chilled water to the wall and ceiling mounted tabular installation. In this arrangement a separate chiller has also been used to provide chilled water at 16°C to the floor mounted tubular installation. A dedicated outdoor air system is also coupled for dehumidification and ventilation purpose. The radiant cooling system integrated with cooling tower to provide chilled water to the wall mounted tabular installation and a separate chiller is used to provide chilled water at 16 °C to the floor and ceiling mounted tabular installation. A dedicated outdoor air system is also coupled for dehumidification and ventilation. A conventional all-air system has also been simulated as a baseline to compare these configurations for assessing the energy saving potential.

The energy consumption globally has been increasing drastically the past decades, mainly due to the population growth and the industrial and technological progress. In order to address this issue, the European Union has launched several directives to decrease energy use, increase energy efficiency and increase use of renewable energy sources. The aim is that by 2020 all new buildings should be nearly zero-energy buildings. A solution that could contribute to this is the combination of photovoltaic panels for the production of electricity and phase change material (PCM) for the reduction of peak cooling demand.

In the present simulation study, the coupling of nighttime radiative cooling with PCM for cooling an office room was investigated. For cooling water through nighttime radiative cooling two types of solar panels were utilized, an unglazed solar collector and photovoltaic/thermal (PV/T) panels. Apart from cold water for space cooling, the installation was capable of providing domestic hot water from both types of panels and electricity from the PV/Ts. This system was simulated for the period from 1^st of May until 30^th of September, under the weather conditions of Copenhagen (Denmark), Milan (Italy) and Athens (Greece).

In Athens and Milan the operative temperature was within the range of Category III of EN 15251 (23 – 26^oC, 73.4 – 78.8^oF) for 81% and 83% of the occupancy period respectively, while in Copenhagen it was within the range only for 63%. Furthermore, the percentage of PCM used at the end of the occupancy period was 86%, 81% and 80% for Copenhagen, Milan and Athens, respectively. Nighttime radiative cooling provided for Copenhagen 61%, for Milan 36% and for Athens 14% of the cooling energy required for discharging the PCM. Furthermore, the average cooling power per unit area provided by the PV/T panels was 43 W/m² for Copenhagen, while for Milan and Athens it was 36 W/m² and 34 W/m², respectively. The cooling power of the unglazed solar collector was negligible. Finally, the total electricity produced in Copenhagen for the simulated period was 371 kWh, while for Milan and Athens it was 380 and 439 kWh, respectively.

It was concluded that the nighttime radiative cooling can be a satisfying solution for providing space cooling to office buildings. The performance of the installation could be improved by implementing a solar shading system and a more precise control strategy.